1. Field of the Invention
This invention relates to a device for pulling a silicon single crystal and more particularly to a device for pulling a silicon single crystal so constructed as to preclude deposition of an unwanted matter on internal members of the device abhorring defilement over a long period of pulling work, elongate the duration of continuous use of the internal members, and simplify the works of cleaning and disassembly of the device.
2. Description of the Prior Art
It is well known that semiconductor elements are produced from substrates which are obtained from a high-purity silicon single crystal bar. The substrates to be used as starting materials for semiconductor integrated circuits, for example, are produced by slicing the high-purity single crystal bar with a diamond blade and subjecting the slices to lapping, etching with a chemical reagent, and polishing to a mirror finish.
In recent years, the advance of semiconductor technologies has come to urge the necessity accurately controlling the dopant concentration, the concentration of impurities other than the dopant, and the density of lattice defects. As a result, a desire has come to be expressed to develop a device to pull the semiconductor single crystal which is capable of effecting the accurate control mentioned above and, at the same time, enabling this accurate control to be carried out continuously over a long time and obviating the necessity of subjecting the device to the works of cleaning and disassembly to the fullest possible extent.
It has been customary to date to adopt the Czochralski method for the work of pulling silicon single crystals. One exmple of the use of this method is illustrated in FIG. 2. In FIG. 2, 1 stands for a crucible for accommodating a molten silicon mass 2, 3 for a heater disposed round the periphery of the crucible 1, 4 for an insulating shield disposed round the periphery of the heater 3, and 5 for a silicon single crystal being pulled.
In the production of a silicon single crystal in accordance with the Czochralski method, a crucible made of quartz is used as the crucible 1. It has been known that oxygen is dissolved out of the quartz crucible, passed into the molten silicon mass, and eventually incorporated in the silicon single crystal.
This phenomenon occurs because the thermal convection of the molten silicon mass accommondated in the quartz crucible and the forced convection arising from the rotation of the crucible in the process of pulling cause the molten silicon mass to act on the quartz crucible and induce a reaction between SiO.sub.2 and Si, the substances forming the quartz crucible, this reaction gives rise to SiO, this SiO is engulfed temporarily in the molten mass, and the greater part of the engulfed SiO vaporizes from the surface of the molten mass and the remainder thereof passes into the silicon single crystal to be permanently settled therein.
This SiO increases the interstitial oxygen concentration of the silicon single crystal being formed and, in some cases, forms a cause for defects and, in other cases, manifests a gettering effect. Thus, the concentration of the oxygen engulfed in the crystal must be controlled with high accuracy.
When a pulling chamber 6 is formed in a perfectly closed structure, the SiO which vaporizes from the molten silicon mass is re-dissolved into the molten silicon mass and momentarily changes the SiO concentration within the pulling chamber 6. Thus, it becomes difficult to pull a single crystal having a fixed oxygen concentration. In the circumstances, the practice of keeping the SiO concentration in the pulling chamber 6 at a constant level by introducing an inert gas 7 into the pulling chamber 6 and causing the inert gas which is departing from the pulling chamber 6 to entrain the SiO emanating from the molten mass in the pulling chamber 6 is now in vogue.
In the conventional device for pulling a silicon single crystal illustrated in FIG. 2, the inert gas 7 is introduced downwardly through a pulling chamber 6, brought into contact with the surface of the molten silicon mass 2, then passed through a gap 9a formed between the crucible 1 and the heater 3 and a gap 9b formed between the heater 3 and the insulating shield 4, and released via an outlet hole 10 and a valve 11 and out of the system by means of a vacuum pump 12. In this case, a condensate originating in the SiO is deposited in a large amount on the surfaces of the heater 3, and the insulating shield 4.
Since the heater 3 and the insulating shield 4 are made of graphite, they are deteriorated by the deposition thereon of the condensate originating in the SiO or they must be treated for removal of the deposited condensate after service. This treatment entails the disadvantage that the device must be disassembled and reassembled at the expense of much time and labor.